In an existing negative electrode of a lithium secondary battery, for instance, natural graphite powders, artificial graphite powders obtained by graphitizing cokes, artificial graphite powders obtained by graphitizing organic polymers, pitch and so on, graphite powders obtained by pulverizing these, spherical graphite powders obtained by graphitizing mesophase carbon and so on can be used. The graphite powders are mixed with an organic binder and an organic solvent to form a graphite paste. The graphite paste is coated on a surface of a copper foil, followed by drying the solvent, and thereby used as a negative electrode for a lithium secondary battery.
For instance, as shown in Japanese Examined Patent Application Publication No. 62-23433, the use of graphite in a negative electrode enables to overcome a problem of internal short-circuiting owing to dendrite of lithium and thereby the cycle characteristics are improved.
However, in the natural graphite having developed graphite crystals, a bonding between graphite layers is cleaved owing to pulverization, since the interlayer bonding force of a crystal in a C-axis direction is weaker than a bonding force in a plane direction of the crystal, and thereby so-called scaly graphite powders large in the aspect ratio result. Since the scaly graphite is large in the aspect ratio, when it is kneaded together with a binder and coated on a current collector to form an electrode, the scaly graphite powders orient in a plane direction of the current collector. As a result, not only a charge and discharge capacity and the rapid charge and discharge characteristics are likely to be deteriorated, but also, internal destruction of the electrode is caused owing to expansion and contraction in a C-axis direction generated by repetition of absorption and release of lithium to the graphite crystal, and thereby the cycle characteristics are deteriorated. In addition, when density of the negative electrode is set at 1.45 g/cm3 or more, lithium becomes difficult to be absorbed and released by negative electrode graphite, resulting in deteriorating the rapid charge and discharge characteristics, a discharge capacity per unit weight of the negative electrode and the cycle characteristics.
On the other hand, in a lithium secondary battery, it is expected that energy density per unit volume can be made larger by the higher density of the negative electrode. In this connection, a negative electrode that is less deteriorated in the rapid charge and discharge characteristics and the cycle characteristics when the density of the negative electrode is made higher in order to improve the energy density per unit volume of the lithium secondary battery is in demand.
The invention, in view of the above situations, intends to provide a negative electrode suitable for a lithium secondary battery excellent in the rapid charge and discharge characteristics and cycle characteristics and a negative electrode suitable for a high capacity lithium secondary battery.